girder vs purlin

girder

noun
  • A beam of steel, wood, or reinforced concrete, used as a main horizontal support in a building or structure. 

  • One who girds; a satirist. 

purlin

noun
  • A longitudinal structural member bridging two or more rafters of a roof. 

How often have the words girder and purlin occurred in a corpus of books? (source: Google Ngram Viewer )